GPS altitude vs altimeter altitude

Chris W wrote:
There is still the possibility that something goes wrong with the
autopilot and sends the plane down the wrong side of the highway at 3
feet off the ground. To virtually eliminate this type of situation, I
plan on having a completely independent system that will kill the engine
and put the plane in a slow flight configuration should the altitude
drop below a given amount. Granted that could also fail but the chances
of both the autopilot and that system failing is very remote, that "fail
safe" system would be pretty small and light and could be made redundant
and possibly include a the deployment of a parachute. All this said I
am open to other ideas to make it safer.

What is the purpose of the project? Why couldn't it be done in a way
where the model remains within sight at all times - say by flying a
pre-established figure-8 course or something similar, and having a
manual override capability if it's observed to stray from the planned
course? Your current plan certainly appears to violate the model
airplane safety guidelines which state that "10. The operator of a
radio-controlled model aircraft
shall control it during the entire flight, maintaining visual contact
without enhancement
other than by corrective lenses that are prescribed for the pilot. No
model aircraft shall be
equipped with devices which allow it to be flown to a selected location
which is beyond
the visual range of the pilot."
http://www.modelaircraft.org/PDF-files/105.PDF